Wide gap between breasts. What is the best implant choice? (Photo)

It is important for patients to understand that augmentation will "augment" all aspects of your anatomy. The spacing of the footprints of the breast on the chest wall will vary from one individual to the next. Widely spaced breast will appear more widely spaced and vice versa. 
The spacing (cleavage) is defined by the pocket. The pocket is defined by the plane of augmentation.When placed submuscular, the medial/middle limit is the sternal origin of the muscle. Wide sternum = greater separation of the breasts = less dramatic cleavage. This space can be narrowed (with some risk of symmastia) when the devices are placed subglandular or "over" the muscle. There is no muscle obstructing a medial push, however, there is a higher risk of over-dissection and mid-line migration of the implant (the dreaded "uni-boob").
Based upon your photos, you appear to be a reasonable candidate for augmentation. A submuscular augmentation may accentuate the wide set nature of your breasts. With that in mind, you may opt for subglandular placement (which will allow for a medial placement). However, one must understand that purposely forcing the implant centrally will not allow for it to be centered on your breast mound.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Thank you for the question and pictures. You are correct: every patient undergoing breast augmentation surgery should understand that their anatomic starting point will limit the outcome of breast augmentation surgery.   In your case specifically, your anatomic starting point will to some degree determine how much space will be present between your breasts after breast augmentation surgery.  Other factors involved include careful breast implant pocket dissection and careful selection of breast implant size/profile.  Remember (when it comes to breast implant pocket dissection and achievement of ideal breast implant cleavage) that each nipple/areola complex must be centered on each breast mound.
Ultimately,  careful selection of plastic surgeon, careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, and computer imaging) will be critical. 
Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. For example, I have found that the use of know words such as “natural” or "full C to D cup” etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery, after the use of temporary intraoperative sizers.
I hope this helps. Best wishes for an outcome that you will be very pleased with.